The present invention relates to wastewater treatment apparatus, and more particularly to a continuous wastewater treatment apparatus incorporating a floating clarifier.
Wastewater from municipal sewage or industrial waste systems is treated prior to release to the environment to meet strict government standards for water quality. This treatment often includes biological treatment processes to degrade the organic contaminates in the wastewater. Biological treatment involves balancing several conditions--for example, the amount of organic waste, the amount of microbes, and the amount of oxygen--so that the microbes convert the organic waste to carbon dioxide and more microbes.
Biological treatment systems include: (1) a biological reactor, (2) a clarifier, and often (3) equalization capacity. The biological reactor includes a tank and an aeration device. Many different aeration devices are known in the art, including coarse-bubble diffusers and impeller draft tubes. In the reactor, the wastewater is aerated so that the microbes suspended in the wastewater consume the organic matter.
After biotreatment, the biologically treated wastewater is fed to a clarifier. The clarifier is a device for settling or separating biological solids from the biologically treated wastewater to produce clarified effluent. The clarifier also concentrates the suspended solids--which includes the microbes--in an underflow stream from the bottom of the clarifier. The underflow returned to the biological reactor is known as "return activated sludge" (RAS). The portion of these solids wasted from the system are known as "waste activated sludge" (WAS). The clarifier typically includes a tank designed to provide sufficient surface area so that the suspended solids in the biologically treated wastewater can settle and concentrate. Several types of clarifiers are known in the art; any geometric shape can be used, for example, circular, square, and rectangular. A variety of sludge removal mechanism exist, including hopper-bottom clarifiers and mechanical clarifiers. The underflow solids from the clarifier are returned to the biological reactor. WAS is removed from the system for further processing and/or disposal, as is known in the art. The clarified, biologically treated wastewater ("treated wastewater") is then discharged for further treatment or directly to the receiving stream.
Equalization refers to capacity of the wastewater treatment facility to store wastewater feed (organic loading) surges above the design capacity until the system can treat the wastewater during subsequent, less-than-capacity demands. Flow equalization may be used to improve clarification ("hydraulic equalization"). Equalization typically involves retrofitting an existing wastewater treatment facility by installation of additional tankage prior to aeration (biological reactor).
It is known to provide equalization by designing a biological treatment unit with built-in equalization capacity. This combination is known in the art as a "variable depth reactor" (VDR), as described in Scroggins & Deiters, Wastewater Cleanup: Put Activated-Sludge Treatment to Work, Environmental Engineering World, November-December 1995, at 22. The wastewater can flow to the VDR at varying rates, yet biotreated wastewater can be drawn for clarification at a consistent, continuous rate. However, the VDR requires a separate clarification unit, which adds to the cost of the treatment system and increases its complexity of operation.
It is also known to utilize a single tank for biological treatment and clarification using batch (non-continuous) processing. This system is known in the art as a "sequencing batch reactor" (SBR). A "batch" of wastewater is fed to the SBR tank, where the wastewater is aerated to effect biological reaction and then settled for clarification. A portion of the settled solids is wasted directly from the SBR tank. Typically, a wastewater system using SBR technology cycles the wastewater flow stream between two SBR tanks in order to provide wastewater treatment that is more continuous than a single SBR tank. In tandem operation, one SBR tank in effect provides equalization capacity while the other SBR tank processes the wastewater.
SBR technology has several disadvantages associated with batch processing. First, the entire wastewater flow to an SBR tank must be discharged after treatment in a fraction of the time that is available for loading the tank. This increases the size and cost of discharge facilities and piping relative to a continuous process. Also, since the aeration requirements must be met in a fraction of the time that would be available for continuous operation, the aeration equipment must have a much higher capacity than that of a continuous aeration process.